1. Field of the Invention
The present invention relates to a semiconductor device including a plurality of cells.
2. Description of the Related Art
Conventionally, an insulated gate transistor such as a power metal-oxide semiconductor field effect transistor (power MOSFET) and an insulated gate bipolar transistor (IGBT) used for a power device is made of an aggregation of a plurality of cells formed in a semiconductor substrate. In a semiconductor device including the insulated gate transistor made of the aggregation of the cells, the cells may be damaged if an overcurrent flows due to a short. Thus, the semiconductor device is required for deactivating the insulated gate transistor so that the overcurrent does not flow. In a case where the insulated gate transistor is used for controlling a rotation of a motor, the semiconductor device is required for controlling an electric current with a high degree of accuracy. In order to restrict a damage due to the overcurrent or control the electric current, a part of the cells may be used as a sensing cell for detecting the electric current as described, for example, in JP-A-2-66975, JP-A-8-8422, and U.S. Pat. No. 7,385,250 (corresponding to JP-A-2006-351985).
A semiconductor device 90 according to an example of the related art includes an N channel vertical power MOSFET made of an aggregation of a plurality of cells, as illustrated in FIG. 8. The aggregation of the cells is formed in a semiconductor substrate 10. The semiconductor substrate 10 includes a first N type (N+) semiconductor layer 1 made of a silicon substrate. The first N type semiconductor layer 1 functions as a drain region D. On the first N type semiconductor layer 1, a second N type (N) semiconductor layer 2 made of an epitaxial silicon layer is formed. An impurity concentration of the second semiconductor layer 2 is less than an impurity concentration of the first N type semiconductor layer. The second N type semiconductor layer 2 functions as a drift layer. On the second N type semiconductor layer 2, a P type semiconductor layer 3 made of an epitaxial silicon layer is formed. The P type semiconductor layer 3 functions as a channel-forming layer.
At a surface portion of the P type semiconductor layer 3, N type (N+) regions 4 are selectively formed. Each of the N type regions 4 function as a source region. At the surface portion of the P type semiconductor layer 3, P type regions 3a are also formed to be adjacent to the N type regions 4. Each of the P type regions 3a is coupled with a corresponding source electrode in common with the adjacent N type regions 4. Each of the P type regions 3a function as a contact region for fixing an electric potential at the P type semiconductor layer 3. Insulated gate electrodes 7 penetrate the P type semiconductor layer 3 so as to be adjacent to the N type regions 4. Each of the insulated gate electrodes 7 includes a trench, an insulating layer 5 disposed on an inner wall of the trench, and an embedded polysilicon 6 filled in the trench through the insulating layer 5. The N type regions 4, the P type regions 3a, and the insulated gate electrodes 7 are arranged in a stripe repeating pattern in a plane of the semiconductor substrate 10.
Each of the cells in the semiconductor device 90 is formed as a configurational repeating unit and has a diffusion structure similar to each other. The cells include main cells C1m and sensing cells C1s. The main cells C1m are provided for supplying an electric current to a load. The sensing sells C1s are provided for detecting the electric current that flows in the main cells C1m. The number of the main cells C1m is larger than the number of the sensing cells C1s. On a rear-surface side of the semiconductor substrate 10, the main cells C1m are coupled with a drain terminal D in common with the sensing cells C1s. In addition, on a front-surface side of the semiconductor substrate 10, the main cells C1m are coupled with a gate terminal G in common with the sensing cells C1s. The source electrodes of the main cells C1m are coupled with a main source terminal Sm. The source electrode of the sensing cells C1s is coupled with a sensing source terminal Ss.
The semiconductor device 90 can be coupled, for example, as an equivalent circuit illustrated in FIG. 9. The gate terminal G of the main cells C1m and the sensing cell C1s is coupled with a gate driving circuit. On a higher potential side, the drain terminal D of the main cells C1m and the sensing cell C1s is coupled with a power source having a voltage of Vdd through a load L. The main source terminal Sm is directly coupled with a ground GND. The sensing source terminal Ss is coupled with the ground GND through a resistor R1. The semiconductor device 90 detects an electric potential of the resistor R1, i.e., an electric potential of the sensing source terminal Ss. Then, the semiconductor device 90 compares the electric potential of the resistor R1 with a reference potential Ref, for example, the ground GND, using a comparator. Thereby, a main current Im that flows in the main cells C1m is detected, and the gate voltage of the insulated gate transistor is feedback-controlled, for example, by using a gate driving circuit.
In the semiconductor device 90, the first N type semiconductor layer 1 used as the drain region and the second N type semiconductor layer 2 used as the drift layer are shared by the main cells C1m and the sensing cell C1s. Thus, the main source terminal Sm for the main cells C1m and the sensing source terminal Ss for the sensing cell C1s are separately provided, and the resistor R1 is coupled with the sensing source terminal Ss. A sensing current Is that flows in the sensing cell C1s is set to be about thousandth part of the main current Im that flows in the main cells C1m. That is, the number or an area of the sensing cell C1s in the semiconductor device 90 is set to be about thousandth part of the number or an area of the main cells C1m. If the resistor R1 is coupled with the main source terminal Sm in which the main current Im flows, an electric current loss increases. Thus, the resistor R1 is coupled with the sensing source terminal Ss in which the sensing current Is set to be about thousandth part of the main current Im flows. Therefore, the semiconductor device 90 can detect the main current Im while reducing the electric current loss. The semiconductor device 90 can restrict a damage due to an overcurrent and can control the main current Im by detecting the main current Im. Although a structure of each semiconductor device described in JP-A-2-66975, JP-A-8-8422, and U.S. Pat. No. 7,385,250 is different from a structure of the semiconductor device 90 illustrated in FIG. 8, the each semiconductor device detects an electric potential of a resistor or an electric potential of a sensing source terminal for restricting a damage due to the overcurrent or controlling a main current, in a manner similar to the above-described method.
In a transistor, when a gate-source voltage VGS exceeds a threshold voltage Vth, the transistor is activated and a drain current ID increases, as illustrated in FIG. 10. For example, the threshold voltage Vth is in a range from about 0.5 V to about 0.6 V, and the gate-source voltage VGS for activating the transistor is set to be about 1 V shown by the dashed-dotted line A1 in FIG. 10.
In the semiconductor device 90, when the transistor is activated, an electric potential equal to the product of the sensing current Is and the resistance R1 generates. The semiconductor device 90 detects the overcurrent by comparing the electric potential of the resistor R1 and the reference potential Ref using the comparator, for restricting a damage due to the overcurrent. When the transistor is activated and the above-described potential generates, the gate-source voltage VGS of the sensing cell C1s decreases as shown by the dashed-two dotted line A2 in FIG. 10 and results in difference between the gate-source voltage VGS of the main cells C1m and the gate-source voltage VGS of the sensing cell C1s. Therefore, a current density of the main cells C1m is different from a current density of the sensing cell C1s, and the semiconductor device 90 is difficult to detect the main current Im with a high degree of accuracy.